1. Field of the Invention
The present invention relates to a sequential decoder for decoding a systematic code.
The systematic code is composed of signal bits and the parity bits formed from the signal bits. In the sequential decoding for decoding the systematic code, a path metric value of one path in a tree-formed code and the received code sequence is calculated, and when the path metric value surpasses a certain threshold value, the data bits are decoded. When the path metric value does not reach a certain threshold value, it is regarded as having entered an erroneous path and a search is made for a correct path, and the data bits in the correct path are decoded.
2. Description of the Related Arts
The sequential decoding system for error correction in convolutional codes is a decoding system for error correction which estimates the most likely decoded data sequence by a trial and error method. In an ideal condition, the decoding system can obtain a correct decoded data sequence as accurately as with a maximum likelihood decoding (Viterbi decoding). This system differs from the maximum likelihood decoding in that, if the constraint length of the convolutional code is lengthened, the circuit scale of decoders is not remarkably enlarged, and thus is advantageous in that the use of a code having a long constraint length enables an improved error correction performance to be obtained by using a long constraint length. Accordingly, this system is specifically suited for use in a satellite communication system or a space probe and the like.
A Fano algorithm is well known in actual sequential decoding, but the Fano sequential decoder has an unavoidable problem in that a degradation of the quality of the channels and an increase of errors cause an increase in the repetition rate of a trial and error method and reduce the decoding speed, and accordingly, the concentration of errors at the channel causes a buffer overflow and delays the decoding.